Animation Nodes - Animating transparency with wait times
First, you get the length of the current text being displayed and you subtract it from the relative time like so:



This will give you the time starting from when the text got fully displayed. Next, you may subtract another constant to delay the fading effect and divide the result to slow it down. The result is then clamped to the 0-1 range and is used as the factor in a material:



Note that the delay and speed constant are related to your time constant (The value of the divide node after the time info node). So a delay of 5 actually means a delay of 5m where m is the time constant.